FFmpeg outputs invalid ASS files

Description

ffmpeg -i test.sub out.ass produces an ASS file with this header:

[Events]
Format: Layer, Start, End, Style, Text

The "Format:" line, as well as all events following it, is semi-invalid. While it does look like the ASS format is flexible about the headers, and libass actually parses this, it's not accepted by most other ASS readers, such as the (very important) vsfilter.